1. Field of the Invention
The invention relates generally to a semiconductor based structure for an active tandem solar cell transitioning from one semiconductor material composition to another by the use of one or more transition layers comprising one or more rare earth compounds enabling control of the stress field between layers and providing means for up and/or down conversion of incident radiation.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As a complementary approach to single or multiple junction solar cells where specific materials are matched to discrete portions of the solar spectrum, wavelength conversion works on the principle of moving parts of the spectrum to the wavelength band of a particular junction cell. For example it is widely accepted that a single junction, single crystal silicon solar cell has an optimum performance in the wavelength range 500 to 1,100 nm, whilst the solar spectrum extends from below 400 nm to in excess of 2500 nm. A tandem cell enables a device to convert a larger portion of the solar spectrum with minimal increase in size and cost.
Rare earths, the lanthanide series, have long been known for the unique optical properties in which the incomplete, 4f shells exhibit multiple optical transitions many of which lie within the solar spectrum. An example of some of these optical transitions are: Er: 410, 519, 650, 810, 972, 1,529 nm; Yb: 980 nm; Tb: 485 nm
As used herein a rare earth, [RE1, RE2, . . . REn], is chosen from the lanthanide series of rare earths from the periodic table of elements {57La, 58Ce, 59Pr, 60Nd, 61Pm, 62Sm, 63Eu, 64Gd, 65Tb, 66Dy, 67Ho, 68Er, 69Tm, 70Yb and 71Lu} plus yttrium, 39Y, and scandium, 21Sc, are included as well for the invention disclosed.
As used herein a transition metal, [TM1, TM2 . . . TMn], is chosen from the transition metal elements consisting of {22Ti, 23V, 24Cr, 25Mn, 26Fe, 27Co, 28Ni, 29Cu, 30Zn, 40Zr, 41Nb, 42Mo, 43Tc, 44Ru, 45Rh, 46Pd, 47Ag, 48Cd, 71Lu, 72Hf, 73Ta, 74W, 75Re, 76Os, 77Ir, 78Pt, 77Au, 80Hg}; additional ones are found in the technical literature and known to one knowledgeable in the art. Silicon and germanium refer to elemental silicon and germanium; Group IV, Groups III and V and Groups II and VI elements have the conventional meaning. As used herein all materials and/or layers may be present in a single crystalline, polycrystalline, nanocrystalline, nanodot or quantum dot and amorphous form and/or mixture thereof.
In addition, certain of these rare earths, sometimes in combination with one or more rare earths, and one or more transition metals can absorb light at one wavelength (energy) and re-emit at another wavelength (energy). This is the essence of conversion; when the incident, adsorbed, radiation energy per photon is less than the emission, emitted, energy per photon the process is referred to as “up conversion”. “Down conversion” is the process in which the incident energy per photon is higher than the emission energy per photon. An example of up conversion is Er absorbing at 1,480 nm and exhibiting photoluminescence at 980 nm.
One concept is disclosed in U.S. Pat. No. 3,929,510; more recent work in this field has mainly focused on the addition of rare earths to phosphorescent compounds. The historical approaches however add a conversion layer to either a completed solar cell or module with the majority also requiring an additional reflective component to return the converted spectrum back into the cell so that it can contribute to the generated photocurrent. U.S. Pat. No. 7,184,203 discloses up and down conversion with a rare earth compound comprising a rare earth element and at least one other element selected from chalcogens, halogens, nitrogen, phosphorus and carbon; wherein the rare earth compound is not mixed with compounds containing other rare earth elements and wherein the rare earth compound is irradiated at a sufficient intensity to heat the rare earth compound to facilitate electronic transitions. U.S. Pat. No. 7,184,203 does not teach or suggest using a rare earth compound in conjunction with a photovoltaic device; U.S. Pat. No. 7,184,203 teaches away from the use of a rare earth compound with relatively low intensity radiation at room temperature for up or down conversion.
U.S. Pat. No. 6,613,974 discloses a tandem Si—Ge solar cell with improved efficiency; the disclosed structure is a silicon substrate onto which a Si—Ge epitaxial layer is deposited and then a silicon cap layer is grown over the Si—Ge layer; no mention of rare earths is made. U.S. Pat. No. 7,364,989 discloses a silicon substrate, forming a silicon alloy layer of either Si—Ge or Si—C and the depositing a single crystal rare earth oxide, binary or ternary; the alloy content of the alloy layer is adjusted to select a type of strain desired; the preferred type of strain is “relaxed”; the preferred deposition method for the rare earth oxide is atomic layer deposition at temperatures below 300° C. While the Si—Ge film is “relaxed”, its primary function is to impart no strain, tensile strain or compressive strain to the rare earth oxide layer; the goal being to improve colossal magnetoresistive, CMR, properties of the rare earth oxide. A preferred method disclosed requires a manganese film be deposited on a silicon alloy first. Recent work on rare earth films deposited by an ALD process indicate the films are typically polycrystalline or amorphous.
U.S. Pat. No. 7,148,417 discloses a first solar cell comprising silicon and a second GaP solar cell. U.S. Pat. No. 6,613,974 discloses a second solar cell comprising silicon-germanium formed on a silicon substrate. U.S. Pat. No. 6,566,595 discloses a tandem solar cell comprising a compound semiconductor and a quantum well layer of a second compound semiconductor. U.S. Pat. No. 6,372,980 discloses a two terminal tandem solar cell comprising InGaAs and GaAs. U.S. Pat. No. 6,340,788 discloses a Si or SiGe solar cell comprising multiple subcells; optionally, transition layer(s) of GaPAs and GaInP are part of the structure. U.S. Pat. No. 6,166,320 discloses a tandem solar cell comprising a first solar cell stacked upon a second solar cell. The cited prior art does not disclose a tandem solar cell comprising a rare earth transition layer of varying composition.